The polyamines, putrescine, spermidine, and spermine are ubiquitous cell components essential for DNA synthesis, cell replication, and normal cell growth. Chemically these compounds are organic aliphatic cations with two (putrescine), three (spermidine), or four (spermine) amino groups that are protonated at physiological pH (Janne, J. (1991) Ann. Med. 23:241-259). The polyamines bind to DNA, RNA and other macromolecules and their biosynthesis has been shown to be temporally linked to and have a role in the regulation of expression of many growth related genes (Eichler, W. (1989) Biol. Chem. Hoppe Seyler 370:451-466; Celano, P.(1989) Biochem. Biophys. Res. Commun. 165:384-390). Depletion of intracellular polyamines diminishes the cells' ability to respond to adverse environmental changes. Cells exposed to heat shock and other stresses normally express Hsp-7 and Hox, which have a protective role, and c-jun and c-fos, which comprise the transcription factor activator protein-1 that is involved in the transcription of a variety of genes. Polyamine depleted mammalian cells express these growth factor gene products at severely diminished levels and have a greatly reduced growth and replication rate (Desiderio, M. A. (1996) Hepatology 24:150-156). Further evidence of the importance of normal polyamine levels for normal cell function is demonstrated by data indicating that over-accumulation of spermidine and spermine in mammalian cells induces apoptosis (Poulin, R. (1995) Biochem. J. 311:723-727).
The intracellular polyamine concentrations are controlled by inducible enzymes of both the biosynthetic and degradative pathways. Putrescine, the precursor of spermidine, is formed by decarboxylation of ornithine, and spermine is derived from spermidine by the addition of aminopropyl residues. In the degradation pathway, spermine is converted to N1-acetyispermidine, and spermidine to putrescine. Both putrescine and N1-acetylspermidine can then be excreted from the cell.
The first step in the degradation pathway is the acetylation of spermine in the N1-position which is catalyzed by the rate-limiting enzyme spermidine/spermine N1-acetyltransferase (Casero, R. A.(1993) FASEB J. 7:653-653). This enzyme is a tightly regulated cytosolic protein whose activity is induced in response to stimuli including toxins, growth factors hormones, polyamines and polyamine analogues, and heat shock (Xiao, L. (1992) Biochem. Biophys. Res. Commun. 187:1493-1502). This highly regulated enzyme is in a metabolic pathway which involves two other highly regulated enzymes, ornithine decarboxylase and S-adenosylmethionine decarboxylase. The degree to which the concentration of these enzymes is regulated emphasizes the importance of precise control of polyamine concentrations. The spermidine/spermine N1-acetyltransferase enzyme is conserved between species, further underscoring the significance of the role of polyamine regulation for normal cell growth (Casero, R. A. (1991) J. Biol. Chem. 266:810-814).
Changes in spermidine/spermine N1-acetyltransferase expression and the corresponding changes in intercellular polyamine levels affect cell growth, viability, and sensitivity to antitumor agents. Increases in spermidine/spermine N1-acetyltransferase expression in E. coli result in reduced cell growth rates and increased sensitivity to the cytotoxic action of the antitumor agent and spermine analogue N1, N12-bis(ethyl)spermine (BESPM); (Parry, L. (1995) Biochemistry 34:2701-2709). In tumor cell lines higher levels of spermidine/spermine N1-acetyltransferase induction by BESPM are correlated with a greater sensitivity to the cytotoxic effects of BESPM. In a comparison of seven human melanoma cell lines, the cells most sensitive to the cytotoxicity of BESPM are those with the greater levels of spermidine/spermine N1-acetyltransferase induction by BESPM (Shappell, N. W. (1992) Anticancer Res. 12:1083-1089). Another spermine analogue, N1, N11-bis(ethyl)norspermine, induces spermidine/spermine N1-acetyltransferase and is specifically cytotoxic to human melanoma xenografts in tumor-bearing mice (Porter, C. W. (1993) Cancer Res. 53:581-586). BESPM treatment inhibits the growth of both hormone-responsive and non-responsive human breast cell lines, human lung cancer cell lines and murine lymphocytic leukemia cells (Davidson, N. E. (1993) Cancer Res. 53:2071-2075; Casero, R. A. (1992) Cancer Res. 52:5359-5363; and Porter, C. W. (1987) Cancer Res. 47:2821-2825). The link between potent induction of spermidine/spermine N1-acetyltransferase and sensitivity of these same cell lines to polyamine analogues represents potential determinants of drug action in particular cancers.
The discovery of the polynucleotides encoding spermidine/spermine N1-acetyltransferase, and the molecule itself, presents the opportunity to investigate their role in regulation of malignant cell growth. The polynucleotide sequence and polypeptides encoding a new spermidine/spermine N1-acetyltransferase protein associated with malignant cellular growth would satisfy a need in the art by providing a new means for the study, diagnosis, prevention, or treatment of specific cancers.